The invention relates to etching oxides, particularly oxides found in integrated circuits.
The need to etch silicon dioxide comes up frequently in the fabrication of integrated circuits and photonics. Typically, HF which is dangerous to people and an environmental hazard is used. HF also has technical drawbacks.
For example, silicon nitride is commonly used as an oxidation mask in processes termed xe2x80x9cLOCOSxe2x80x9d (localized oxidation of silicon). In a LOCOS process, areas covered by nitride are not oxidized although the nitride does convert to oxide albeit at a slow rate. Following the oxidation process, current practice is to remove the oxide formed on the nitride by immersion in HF acid. This is done to allow the subsequent removal of the nitride in phosphoric acid. This removal of the oxide from the nitride surface unavoidably and undesirably etches the oxide formed in the areas that were intentionally oxidized, such as the field oxide.
Also, consider an optical device, such as a micro electromechanical system (MEMS) mirror array. It is often desired to make one or more regions in a silicon dioxide substrate where the silicon dioxide is a different height than the substrate. This has been done by timed etch, but better and more controllable results are had by providing an HF resistant etch stop layer such as SiN (silicon nitride) during the deposition of the silicon dioxide to provide a SiO2/SiN/SiO2 sandwich. The top layer is patterned with photoresist and HF or other fluorinated compounds are used to etch down to the SiN. While the SiN may then be etched away locally by phosphoric acid, a layer of SiN remains buried in the unetched SiO2 areas. If these areas are then used to transmit light, provided with a back-surface reflector, undesirable light loss will occur due to the SiN having a different refractive index than the SiO2.
The invention includes a method of etching silicon dioxide, comprising doping a layer of silicon dioxide to form a layer of doped silicon dioxide and etching the doped silicon dioxide layer with phosphoric acid.